The market of fishery products in Japan has recently been on a declining trend due to population decline and the change in food culture. Japanese foods, such as sushi and slices of raw fish (sashimi), however, are spread in the world, and an increase in demand for luxury foodstuffs due to an increase in purchasing power in Asian countries and other areas is expected. In order to transport Japanese fishery products overseas, a transport technique and a freshness retention technique are required.
Fishes with high-fat flesh, typified by salmon, are abundantly consumed outside Japan. In addition, fishes with light-taste and high-lipid flesh are relatively less inhabited outside Japan, and freshwater fishes with such flesh may be consumed, but saltwater fishes with such flesh are less consumed.
On the other hand, fishes with light-taste and high-lipid flesh are abundantly inhabited in Japanese coastal waters, and for example, fishes belonging to the family Carangidae are abundantly inhabited. A representative example thereof is yellowtail. Yellowtail is currently actively aquacultured in Japan (including Ehime Prefecture, Kagoshima Prefecture, and other areas). Therefore, yellowtail is fish species suitable for exporting abroad.
The quality of fish meat is generally evaluated based on appearances (including color and shape), taste, texture, and other features. In particular, the quality of fish meat is evaluated in general retailers, with focusing on appearances easily evaluated. The quality of fish meat is thus usually evaluated by appearances, such as color and shape. The reason is because a labor time is required and it is not easy to confirm taste and texture with respect to each fish meat in order to confirm the freshness of fish meat to be sold in shopfronts. In addition, fish purchasers also often select a commercial product in shopfronts based on the color of the cross section of a slice of fish.
Meanwhile, when fish meat comprises dark colored flesh, like meat of yellowtail, a protein called myoglobin is present in the muscle of fish meat. Myoglobin is a main component constituting the dark colored flesh of fish meat. Since myoglobin is higher in affinity with an oxygen molecule than hemoglobin in blood, myoglobin receives oxygen in blood and releases it in metabolism, in vivo. The oxidation-reduction mechanism of myoglobin is thus constructed in vivo, making it possible to maintain ecological activity.
Normal myoglobin contains a divalent iron ion (Fe2+) in home. An oxygen molecule coordinates thereto to thereby provide oxymyoglobin, and oxymyoglobin is further oxidized to provide metmyoglobin having a trivalent iron ion (Fe3+). Since metmyoglobin is reduced by the enzyme action in fish, in vivo, to provide normal myoglobin containing Fe2+, no metmyoglobin is accumulated therein. After the death of fish, however, a mechanism of donating and receiving of oxygen (oxidation-reduction reaction) is lost to cause the reduction action to be lost. That is, progression of the oxidation reaction (receiving of oxygen) is rapidly increased. Such progression of oxidation is generally referred to as “metmyoglobin formation” (auto-oxidation of myoglobin).
As such metmyoglobin formation progresses, the dark colored flesh in fish meat is changed in color from bright red to brown or gray, causing appearances of fish meat to remarkably deteriorate.
In addition, almost no metmyoglobin formation progresses under a temperature condition of −30° C. or lower. When fish meat including dark colored flesh, such as meat of yellowtail, is transported and/or preserved for a long time in Japan, it is distributed in a low-temperature distribution system (cold chain) by which a temperature of −30° C. or lower can be kept. Only a fresh food which can be distributed in a short time is transported under refrigeration (4° C.).
Cold chains used outside Japan are usually a distribution system at a temperature of −20°. The reason is because distribution of livestock meat accounts for the majority of the distribution outside Japan and such a distribution system sufficiently functions at −20° C. Such cold chains kept at a temperature of −2.0° C. can hardly suppress metmyoglobin formation in fish meat.
It is not easy to introduce cold chains allowing fish meat to be distributed at −30° C. or lower in such countries where they have not been introduced.
One measure for suppressing deterioration in appearances of fish meat where metmyoglobin formation easily occurs, in transport of such fish meat by the cold chain at −20° C. or higher, comprises transporting fish meat with the atmosphere in a packaging container being replaced with CO (carbon monoxide) gas when packaging the fish meat. CO can more strongly coordinate to myoglobin than oxygen. Thus, oxidation is suppressed and myoglobin (carboxymyoglobin) to which CO coordinates displays a bright red color.
Fish meat having myoglobin, however, when treated with CO, is kept red even after a long lapse of time after the treatment and does not deteriorate in appearance, and therefore causes consumers to misidentify such fish meat as fresh fish meat and can also cause food poisoning to occur. Therefore, the treatment of fish meat with CO has been prohibited under the Food Sanitation Act in 1994 in Japan. The treatment is also prohibited currently in Europe, and is also scheduled to be prohibited in USA. In addition, destruction of cells is also caused in such fish meat, and therefore such fish meat deteriorates in freshness while looking (color) thereof being fresh.
As described above, color is important as a discrimination index of freshness of a myoglobin-containing food, such as fish meat. The freshness of a myoglobin-containing food, however, needs to be evaluated comprehensively in terms of not only color, but also form, texture, odor, the amount of drip, K-value described later, and other viewpoints.
The K-value is known as an index for evaluation of freshness deterioration of a myoglobin-containing food, such as fish meat, by an objective numerical. Adenosine triphosphate (ATP) as an energy source is present in fish meat and such a substance is rapidly decreased by an enzyme action in the muscle after the death of fish to be thereby sequentially decomposed to adenosine diphosphate (ADP), adenosine monophosphate (AMP), and inosinic acid (IMP) (an umami component), and further decomposition progresses to allow inosine (HxR) and hypoxanthine (Hx) to accumulate. ATP, ADP, AMP and IMP are more accumulated in fresh fish. HxR and Hx are more accumulated in unfresh fish. The K-value refers to the value expressed in percentage of the total amount of inosine and hypoxanthine in the total amount of ATP-related compounds, as represented by (Formula A) proposed by Tsuneyuki Saito (professor emeritus at the University of Hokkaido) in 1959 (Non Patent Literature 2). The K-value represents the following numerical values: 10% or less in fish just after death, approximately 20% in slices of raw fish, and about 60% at initial decay. The rate of decrease in freshness, which can be evaluated based on such values, differs depending on the type of fish, and, in general, is high in the case of codfish and low in the case of white fish, such as porgy and flatfish.K-value (%)=HxR+Hx/[ATP+ADP+AMP+IMP+HxR+Hx]×100  (A)
Preventing oxidation of fish meat is required to prevent discoloration and freshness deterioration of fish meat comprising myoglobin. Conventional methods for preventing oxidation of fish meat without any CO treatment are roughly classified to two methods: (1) avoiding oxygen and fish meat from being in contact with each other; and (2) preventing oxidation of fish meat.
A representative method as the method (1) includes vacuum packing fish meat. It is, however, difficult to completely expel oxygen from fish meat. Expelling oxygen excessively can cause fish meat to be compressed and collapsed by atmospheric pressure. In addition, only vacuum packing can highly likely cause oxygen to enter into a pack through a sealing portion due to impact in distribution and/or long-term distribution.
Examples of the method (2) include:    (2-1) immersing fish meat in an antioxidant solution,    (2-2) mixing fish meat and an antioxidant,    (2-3) freezing fish meat in a specified condition,    (2-4) packaging fish meat by a film containing a chemical agent which supplies an agent acting as a ligand for myoglobin, and    (2-5) providing a fish feed having an antioxidative action to live fish.
The following methods are known as the method (2-1). Patent Literature 1 discloses immersing fish meat in a hinokitiol solution. Patent Literature 2 discloses immersing fish meat in a solution containing hinokitiol and one of kojic acid, ascorbic acid and sodium ascorbate. Patent Literature 3 discloses immersing fish meat in a solution containing trehalose and sodium carbonate or potassium carbonate. Patent Literature 5 discloses a color fading and/or discoloration preventing agent of fish meat, comprising ascorbic acid or sodium ascorbate, ferulic acid and a pH adjuster. Patent Literature 5 discloses a method of immersing fish meat in an aqueous solution of the color fading and/or discoloration preventing agent, and a method of directly adding the color fading and/or discoloration preventing agent to fish meat. Patent Literature 11 discloses immersing chicken in seawater containing a solution of rosemary extracted with warm water. Patent Literature 13 discloses immersing beef cut in an aqueous rosemary solution (rosmarinic acid, carnosic acid, carnosol).
The following methods are known as the method (2-2). Patent Literatures 1 and 2 disclose compounding of an antioxidant, such as hinokitiol, to mince of fish meat. Patent Literature 4 discloses compounding of an antioxidant (reductant), such as ascorbic acid and erythorbic acid, to mince obtained by pulverizing tuna flesh. Patent Literature 5 discloses direct addition of the color fading and/or discoloration preventing agent of fish meat, comprising ascorbic acid or sodium ascorbate, ferulic acid and a pH adjuster, to mince of fish meat. Patent Literatures 9 and 10 disclose, as a reference example, immersing of fish meat cut into dice in an aqueous rosemary extract solution. Patent Literature 12 discloses addition of ascorbic acids to mince of tuna flesh.
The following methods are known as the method (2-3). Patent Literature 6 discloses quickly freezing fish meat at a temperature of −30° C.′ or lower (−50° C. in Examples) to thereby suppress discoloration of fish meat.
The following methods are known as the method (2-4). Patent Literature 7 discloses a food packaging article for suppression of discoloration of a myoglobin-containing food. The food packaging article comprises a food contact layer containing a myoglobin blooming agent, and an oxygen barrier layer. The myoglobin blooming agent is defined in Patent Literature 7 to mean any agent (or precursor thereof) that binds to or interacts with any undenatured myoglobin-containing structure (including, but not limited to, deoxymyoglobin, oxymyoglobin, metmyoglobin, carboxymyoglobin, and nitric oxide myoglobin) present in a fresh meat product to produce or preserve a desired color, such as a red color, indicative of fresh meat. The myoglobin blooming agent includes a nitric oxide-donating compound, a nitrogen heterocycle, a carbon monoxide-donating compound, a sulfur monoxide-donating compound, a nitrous oxide (N2O)-donating compound, an ammonia (NH3)-donating compound and a hydrogen sulfide-donating compound. In Patent Literature 7, it is considered that nitric oxide, a nitrogen heterocycle, carbon monoxide, nitrous oxide (N2O), ammonia or hydrogen sulfide donated by such a myoglobin blooming agent can act as a ligand for myoglobin to maintain the color of myoglobin in fresh meat. Furthermore, Patent Literature 8 corresponds to the International Patent Application whose applicant is the same as that of Patent Literature 7, and discloses a food packaging film comprising a food contact layer containing a nitric oxide-containing compound, and an oxygen barrier layer. Patent Literature 8 discloses, as in Patent Literature 7, a technique where a packaging film compound containing nitrogen oxide interacting as a ligand for myoglobin, such as nitrite, is brought into contact with a myoglobin-containing food to thereby produce a color desirable for the myoglobin-containing food. Patent Literature 8 suggests that a rosemary extract can strengthen the action of the compound containing nitrogen oxide.
As the method (2-5), Non Patent Literature 1 reports that metmyoglobin formation hardly occurs in fish meat taken from young yellowtails to which 0.02% of a tea powder or a food containing 0.02% of a tea powder has been given over about one month. Patent Literatures 9 and 10 also teaches that specified feed given to fish can suppress the change in color tone of fish meat processed to slices of raw fish.
Patent Literature 14 discloses effectiveness of rosmarinic acid contained in a rosemary extract and other compounds, as a deterioration preventing agent for food other products. The literature also discloses a plastic product containing the deterioration preventing agent. Examples of the plastic product include a food packaging material. The proportion of the deterioration preventing agent to be used in the plastic product, however, is merely described to be in a very wide range from 0.0005 to 5% by weight even as the most specific range, and no specific embodiment is described.